The first guy peers into it and says, "Wow! That looks deep."
The second guy says, "It sure does. Let's throw a few pebbles in there and see how deep it is. We'll be able to tell the depth by how long it is before we hear the noise of the pebbles landing."
So they pick up a few pebbles and throw them in and wait. Nothing. There's no noise.
The first guy says, "Jeeez. That is really deep. I know, let's throw one of these great big rocks down there. Those should make a noise."
So they pick up a couple football-sized rocks and toss them into the...